MRI has developed into one of the most versatile and useful tools available in medicine and biology. Despite its wide use as a diagnostic and experimental method, several practical difficulties remain in its implementation. Among these are the high maintenance costs for a typical MRI system. For example, a cryogenic magnet system requires frequent replacement of liquid helium and liquid nitrogen. In addition, in a typical MRI apparatus, the patient is introduced into a cramped cylindrical chamber, leading to anxiety and claustrophobia.
One approach that has been taken to address some of these problems has been the use of permanent magnets as the source of the magnetic field, and many examples of designs of permanent magnet systems adapted for MRI are known in the prior art.
For example, U.S. Pat. No. 5,332,971 teaches and describes a permanent magnet for MRI that utilizes a number of magnetic blocks arranged into concentric rings to produce a homogeneous magnetic field. U.S. Pat. No. 5,623,241 teaches and describes a permanent magnet structure comprising a C-shaped yoke that supports two opposed magnet groupings at the open end of the frame legs. The magnet groupings are single-piece permanent magnets arranged in a parallel, face-to-face relationship, with each magnet having a pole piece positioned on its outer face. U.S. Pat. No. 6,670,877 teaches and describes an improved permanent magnet arrangement for MRI in which opposing magnet groupings face each other across an air gap and which further improves the magnetic field strength and homogeneity by attaching the magnet groupings to backing plates. U.S. Pat. Nos. 7,023,309 and 7,053,743 teach and describe additional refinements of permanent magnet assemblies for MRI in which the presence of soft magnetic material between the yoke and the magnet groupings obviates the need for additional pole pieces.
These designs all suffer from several problems. In all cases, the magnets are permanently positioned, so the active volume is fixed in space, and the body part of interest must be inserted into the active volume. In addition, the arrangements taught in the prior art tend to be large and unwieldy. Thus, there remains a long-felt need for a permanent magnet arrangement suitable for use in an MRI system that is relatively lightweight, open to the air, and in which the active volume can be defined by the user. The present invention, which comprises a mobile permanent magnet above a ferromagnetic facing plate, is designed to answer this long-felt need.